James Cameron's Titanic has a small plot hole with its necklace narrative. In the first act, the older version of Rose (Gloria Stuart) claims that she only wore "The Heart of the Ocean" one time; yet, a flashback scene reveals that she proudly donned Le Coeur de la Mer on more than one occasion. Given the 84-year time gap, Rose may have difficulty remembering the specifics, but she also may have blatantly chosen to block out memories from her Titanic experience.
The diamond necklace moves the storyline forward in Titanic. In the 1996 narrative, Brock Lovett (Bill Paxton) leads an expedition to find Le Coeur de la Mer, and subsequently finds a drawing of a woman wearing the historic relic. From there, older Rose recalls her experiences aboard the RMS Titanic, and the events that led to her wearing the diamond necklace while posing for a man named Jack Dawson (Leonardo DiCaprio). Kate Winslet portrays the 17-year-old Rose in the 1912 timeline.
In Titanic, the mid-movie portrait sequence anchors the narrative, and also marks the moment when Rose and Jack truly fall in love. The first half of the movie explores their friendship aboard the doomed ship, along with Rose's rocky relationship with fiancee Cal Hockey (Billy Zane). The wealthy businessman continuously patronizes Jack in Cameron's film, but still invites him to a first-class dinner: a reward for calming Rose down when she seemed ready to end her life. This all leads to the now-famous portrait scene, in which Rose asks Jack to paint her in the nude. Winslet's character then disrobes and wears nothing but Le Coeur de la Mer. In 1996, older Rose claims that she only wore the necklace that one time, but she actually wore it before when it was given to her by Cal as a gift to calm her nerves.
At the 46-minute mark in Titanic, Rose enjoys some quiet time in her room after being saved by Jack. Cal pays her a visit, and inexplicably doesn't seem to understand the source of his fiancee's "melancholy." He does, however, come with a gift: a 55-carat diamond that was previously owned by Henry XVI, Le Coeur de la Mer. "It's overwhelming," Rose says after Cal puts "The Heart of the Ocean" around her neck and lovingly gazes at her visage in a mirror. Importantly, this moment doesn't take place on the same day of Rose's art session with Jack, but rather at least one day before, which is why some interpret Rose's claim that she "only wore it once" as a plot hole. However, one could argue that because Cal put the jewelry around Rose's neck, she wasn't truly "wearing" it. In Rose's mind, she didn't actively make the choice to wear Le Coeur de la Mer until the next day when she models for Jack.
The necklace later plays an important role in the film's second half, as Cal plots against DiCaprio's character and uses the diamond as a framing device. The controversial ending of Titanic revealed that Rose had "The Heart of the Ocean" all along, which seemingly connects to her claim that she only wore it once. Rose ultimately lets go of the past by throwing Le Coeur de la Mer into the ocean. It's a poignant moment to close the film, and it thematically links to the "I'll never let go" concept that's steadily reinforced throughout the narrative. So, if the diamond necklace represents a positive and lasting memory of Jack, then Rose most likely blocked out any negative associations with Cal. After all, both men died tragically, and the specifics of memories will naturally change over the course of several decades. But even though Rose might be fully aware that Cal originally placed Le Coeur de la Mer around her neck earlier in Titanic, she may truly believe that she really only wore the necklace while posing for Jack.